74 THE QUANTITATIVE METHOD IN BIOLOGY 



In the body of a vertebrate animal we find cells of various 

 kinds ; for instance, nervous, glandular, epidermal, muscular, 

 cartilaginous, ciliate cells, etc. The differences between these 

 cells are strongly marked : if they were Protozoa, they would 

 be classified into different genera, families, etc. Between them 

 there are, however, no hereditary differences (the possibilities 

 are the same in each and all), since they all belong to the off- 

 spring of one egg ; in each of them certain peculiar reactions 

 have resulted in peculiar properties according to the social in- 

 fluences. This example illustrates, perhaps better than any 

 other, the fundamental distinction between hereditary possi- 

 bilities and observable properties. Every biologist knows these 

 facts, but only few realise the importance of the consequences 

 which may be drawn from them. 



§ 56.-^FIRST EXAMPLE : SPIROGYRA {continued) .—In 

 each cell of a Spirogyra two primordia may be recognized : 

 length and breadth. They are easily measurable, the measure- 

 ments being carried out in the direction of two axes (longi- 

 tudinal and transverse).! 



§57.— FIRST EXAMPLE: SPIROGYRA [continued).— 

 The primordia which we have already discerned in a Spirogyra 

 (or a similar plant) may be brought into five groups : 



(i) Number of axes (of segmentation and diferentiation) : 

 one. 



(2) Number of segments (cells = individuals ::i; + i). 



(3) Number of sorts of segments: two (terminal and inter- 

 mediate). 



(4) Form of the two sorts of segments (cylindrical and 

 cylindrical with one rounded extremity). 



(5) Dimensions of the segments (length and breadth). 



All these primordia are measurable (the term cylindrical 

 represents a strictly geometrical notion) except the rounded 

 extremity. (This is not a difficulty from a theoretical stand- 

 point ; it merely depends on our ignorance.) 



We may add to the above list the length and the breadth of 

 the egg (fifth group : dimensions). 



It is possible to go further and to distinguish a number of 

 primordia within the cells ; for instance, the number of chromo- 

 phores, the dimensions of the nucleus, the number of chromo- 

 somes, the physical properties (optical axes, etc. STEIN- 

 BRINK) of the cell- walls, certain chemical properties (presence 

 of tanin in the vacuoles), etc. From a theoretical point of view 



1 It would be interesting to ascertain whether any difference exists between 

 the terminal and the intermediate cells with regard to their length and eventu- 

 ally whether the difference is the same in all the species. 



